Hamas has a new leader. How will that affect the war in Gaza and cease-fire efforts?

Yahya Sinwar’s appointment as the top leader of Hamas formalizes a role he assumed in the early hours of Oct. 7, when the surprise attack into Israel that he helped mastermind ushered in the bloodiest chapter of the Israeli-Palestinian conflict.

He is seen as a hard-liner with closer ties to Hamas’ armed wing than his predecessor, Ismail Haniyeh, who was killed in an explosion in Iran’s capital last month that was widely blamed on Israel and could spark an all-out regional war.

Sinwar...